The invention relates in general to a system that measures the ability of the eye to track an object and in particular the ability of the eye to fixate steadily upon an object while the head is undergoing normal random motion in three axes.
A servo system is one in which the desired response is compared to the actual response and the difference or error is applied to the driving means in such a way as to reduce the error to zero. It is the purpose of this invention to treat the eye-positioning mechanism as a servo system and to measure the quality of this system in much the same way as in an electro-mechanical servo system.
In prior art publications related to human performance, it has been noted that the measurement of alertness required that the subject perform some structural mental activity requiring concentration. However, these techniques are not suitable when the subject is performing a real world task that requires full attention, such as driving a vehicle.
Other techniques such as using infrared or other sensors to passively monitor the activity of the eyelid or eyeball are not satisfactory because they fail to take into account the normal movement of these elements in response to the motion of the head and other normal visual processes. They also fail to provide a rapid indication of visual disfunction.
Another prior art system utilizes eyeblink waveforms for detecting the onset of drowsiness. These systems require a history of eyeblinks in order to detect a deviation from the normal. This limits a rapid response of the system necessary to avoid, for example, vehicular accidents.
The basis of the present invention is to measure and evaluate the motion of the eye with respect to the head. In the alert human being, whenever the eye is open it is fixating on an object or detail of interest. If the head moves while fixating on an object, the perceived image will start to shift which generates a small error. In response to this error, the oculomotor system positions the eye in the precise amount necessary to keep this error near zero so that the image appears stable.
Under the conditions of fatigue, inattention or substance abuse, the observed scene will move or jitter in response to head motion. This is the result of a transient inability to reduce the error to zero.
The present invention is therefore different from known prior art systems in that is based upon servo system principles which can be readily measured and evaluated. Normal visual artifacts, such as the relatively slow motion of the observed object or saccadic motion (the rapid jump made by the eye when moving from one detail to another) are removed during the evaluation.